Several generally similar but mutually incompatible systems are available on the market providing the above facilities. In such a system, typified by the ELECTRIPAK 7 system from Electri-Cable Assemblies Inc., each outlet unit consists of a series of plug together modules, including outlet modules which can be plugged into the assembly in any desired combination to meet local requirements. In practice, the system is less flexible than at first appears, since the ability to change the outlet combination available at any station depends on the availability of stocks of three different types of proprietary simplex outlet modules for interchange purposes. The modules need to be colour or otherwise coded to indicate which supply they are configured to tap.